1. Field of the Invention
The present invention relates to a cleaning roller and cleaning apparatus used with an image forming system utilizing an electrostatic process, such as an electrostatic copying machine, an electrostatic printer or the like.
2. Related Background Art
In conventional image forming systems wherein a process for transferring a toner image electrostatically formed on an image bearing member onto a transfer material as a recording sheet is repeated, in order to obtain a good image, whenever each transferring operation is finished, it is indispensable to adequately remove residual toner not transferred onto the transfer material and remaining on the image bearing member, paper powder generated from the transfer material, eductions such as rosin and/or talc, and foreign matter adhered to the image bearing member such as corona product generated due to the presence of high pressure members in the system.
As the cleaning means for removing such foreign matters and the like, in the past, a cleaning apparatus including a cleaning blade made of elastic material such as rubber for removing mainly the residual toner, and an elastic cleaning roller made of urethane rubber or the like in sliding contact with the image bearing member for removing the above-mentioned foreign matter and the like has been proposed.
FIG. 6 shows a typical example of such conventional cleaning apparatus 2 which is arranged in parallel with and in the vicinity of an image bearing member 1 extending in a direction perpendicular to a plane of FIG. 6 and rotatable in a direction shown by the arrow A. A cleaning blade 3 attached at its one end of the cleaning apparatus has the other free end pressed against a surface of the image bearing member 1 and serves to remove residual toner remaining on the image bearing member (which was not transferred onto a transfer material in a transfer station (not shown)).
Incidentally, around the image bearing member 1, image forming means such as a charger, a developing device and the like (all not shown) are arranged. Further, at an upstream side of the cleaning blade 3 with respect to the rotating direction of the image bearing member 1, an elastic cleaning roller 4' is arranged and is pressed against the image bearing member. In operation, the cleaning roller is rotated at a speed of 50%-30% with respect to the rotation speed of the image bearing member to frictionally slide on the latter.
The cleaning roller 4' comprises a core metal 4'c, an outer layer 4'a made of insulation material such as silicone rubber, urethane rubber or the like, and inner layer 4'b made of sponge or elastic insulation material which is the same kind of the material of the outer layer but has lower hardness, so that the pressing force against the image bearing member is appropriately regulated or adjusted. A scraper 5 is urged against the cleaning roller 4' to form a thin toner layer on the outer layer 4a' thereof. In this way, by frictionally sliding the cleaning roller 4' on the image bearing member 1 and by appropriately selecting the materials of the cleaning roller 4' and of the scraper 5 to maintain the cleaning roller at high potential due to the friction between the elements 4' and 5, the residual toner is electrostatically removed from the image bearing member. In order to remove the toner as mentioned above, it is necessary to generate the electric charge on the cleaning roller; and to do so, it is necessary to form the outer surface of the cleaning roller with insulation material having high volume resistance. With this arrangement, the toner and other foreign matter can be removed effectively. However, when the cleaning roller is maintained at a high voltage as mentioned above, if the printing operation for obtaining particularly an image having an intermediate tone is performed, by starting the image forming system again after several tens of minutes has elapsed from when the system was stopped, there arose a problem that a low density area or a white blank area (impression of roller) was generated at a position of the image corresponding to a position where the cleaning roller was engaged by the image bearing member during stoppage of the system.
For example, if an OPC sheet having a negative cleaning feature was used as a photosensitive member and and the cleaning roller having outer and inner layers made of urethane rubber as insulation material was used, after 500 transfer sheets of A4 size have continuously been handled and then the system was stopped for 20 minutes, when the system was operated again to obtain an image having a density of 0.3-0.7, it was found that the white blank of 1-6 mm was generated at a position of the image corresponding to a position where the cleaning roller was contacted with the image bearing member (normally, forming a nip of 2-4 mm).
The reason why the white blank is generated is not always clear, but is considered as follows. That is to say, the negative frictional charge generated during the operation of the cleaning roller maintains the surface of the roller at relatively high voltage after the roller is stopped, with the result that the photosensitive layer of the image bearing member is also charged at the similar voltage. Consequently, when the positive charge is added from the base of the image bearing member, the trapping condition is generated, so that, when the system is operated again after it has been stopped for a certain time, the voltage of the trapped portion cannot be adequately increased. After all, by charging the cleaning roller, the cleaning ability is improved, but the quality of the image is worsened due to the generation of the white blank.